Wear-out stress monitor utilising temperature and voltage sensitive ring oscillators

The authors propose an on-chip wear-out monitoring technique, which is based on monitoring the environmental conditions experienced by a digital circuit. The frequency of the T-sensitive ring oscillator (RO) emulates the wear-out stress strength caused by the temperature conditions based on the model of exponential dependence of the stress on the inverse of temperatures. The frequency of the VT-sensitive RO emulates the stress due to time-dependent dielectric breakdown, which is stressed by voltages as well as temperatures. Thus, the accumulated counts driven by the ROs directly indicate the total wear-out stress that the product has experienced so far. The measured results of a test chip fabricated by 28 nm High-k Metal Gate process confirm the expected dependence of T-/VT-sensitive RO frequencies on temperatures and voltages, enabling the emulation of wear-out. The methodology is presented to estimate the stress amount of various wear-out factors having different thermal activation energies. The proposed wear-out stress monitor would make automotive microcontrollers more reliable when they operate at boosted voltages and elevated temperatures to meet performance requirements of cutting-edge applications such as advanced driver assistance systems.